The present invention generally relates to a rolled copper or copper alloy foil comprising a roughened surface, and in particular relates to a rolled copper or copper alloy foil which has reduced craters, high strength, adhesive strength with the resin layer, acid resistance and anti-tin plating solution properties, high peel strength, favorable etching properties and gloss level, and is also suitable for use in producing a flexible printed wiring board capable of bearing a fine wiring pattern, as well as to a method of roughening the rolled copper or copper alloy foil. The term “craters” as used herein refers to the minute spot-shaped surface irregularities caused by uneven roughening treatment. In addition, the expressions of % and ppm used herein refer to wt % and wtppm, respectively.
Since a high purity copper foil is soft in comparison to copper alloy, there is an advantage in that rolling can be performed easily and it is possible to produce an ultra thin rolled copper foil. From this perspective, high purity copper foil is preferable for use in a flexible printed wiring board required to bear a fine pattern. Meanwhile, a copper alloy rolled copper foil is characterized in that, by adding trace amounts of alloy elements, it is possible to increase corrosion resistance and in particular increase the strength of the copper foil, whereby the body of the copper foil becomes strong and resilient to scratches and breaks. Thus, since it is possible to similarly make the copper foil thin and easily bear a fine pattern, is useful for use in a flexible printed wiring board.
The present invention relates to a rolled copper or copper alloy foil and its roughening method that is suitable for producing a flexible printed wiring board capable of controlling the generation of craters.
In recent years, with the development of miniaturization/integration technology of components to be mounted on semiconductor devices and various electronic chips and the like, a finer pattern is being demanded of print wiring boards to be produced from the flexible printed wiring boards mounted with the foregoing components.
Conventionally used was an electrolytic copper foil subject to roughening treatment in order to improve the adhesive properties with resin, but there were problems in that the roughening treatment caused the significant deterioration in the etching properties of the copper foil, made etching difficult at a high aspect ratio, caused undercuts during the etching process, and made the bearing of a sufficiently fine pattern impossible.
Thus, in order to inhibit the occurrence of undercuts during the etching process and to meet the demands of a fine pattern, a method of moderating the roughening treatment of the electrolytic copper foil, that is, a method of achieving a low profile (reduction in roughness) has been proposed.
Nevertheless, achieving a low profile of the electrolytic copper foil will cause a problem of deteriorating the adhesive strength between the electrolytic copper foil and the insulating polyimide layer. Thus, there are demands of achieving a high-level fine wiring pattern on the one hand, however, there is a problem in that it is not possible to maintain the desired adhesive strength on the other hand, and the wiring will separate from the polyimide layer at the processing stage.
As a solution of the foregoing problem, proposed is a method of using an electrolytic copper foil in which the surface has not been subject to roughening treatment, forming a thin zinc-series metallic layer thereon, and further forming polyamide resin thereon (for instance, refer to Japanese Patent Laid-Open Publication No. 2002-217507).
Additionally proposed is technology for forming a phosphorous-containing nickel plating layer on the electrolytic copper foil for the purpose of preventing undercuts (for instance, refer to Japanese Patent Laid-Open Publication No. S56-155592). However, the surface of the electrolytic copper foil in this case needs to be a rough surface, and the technology at least tolerates such rough surface. In addition, all Examples of Japanese Patent Laid-Open Publication No. S56-155592 form a phosphorous-containing nickel plating layer on the rough surface of the electrolytic copper foil.
Nevertheless, the characteristics required for achieving a sophisticated fine pattern of a copper foil are not limited to the undercuts caused by the etching and the adhesive properties with the resin. Examples of other required characteristics are superior strength, acid resistance, anti-tin plating solution properties, and gloss level.
Nevertheless, it is unlikely that these comprehensive problems have been studied in the past. At present a copper foil suitable for overcoming the foregoing problems has not yet been discovered.
In light of the foregoing circumstances, rolled copper foils formed of high-strength pure copper are being used in order to overcome the problems of the electrolytic copper foil described above.
Generally speaking, a copper foil obtained by additionally performing fine copper plating (so-called “red treatment”) to a rolled copper foil formed of pure copper to increase the adhesive strength with resin and the like is well known.
Copper and cobalt alloy or ternary alloy of copper, cobalt and nickel is usually further plated on this roughened surface for forming a copper foil to be used in a print circuit (refer to Japanese Published Examined Patent Application (Kokoku) No. H6-50794 and Japanese Published Examined Patent Application (Kokoku) No. H6-50795).
In recent years, rolled copper or copper alloy foil with improved strength and corrosion resistance capable of bearing a fine wiring pattern is being proposed in substitute for conventional rolled copper foil. See Japanese Patent Laid-Open Publication No. 2002-241989.
However, when copper is plated to this kind of copper alloy rolled copper foil and minute copper particles are formed, defects referred to as craters will arise. These craters are minute spot-shaped surface irregularities caused by uneven roughening treatment that become holes (spots) during the treatment, in other words, craters are defects in which copper particles are not formed or are formed sparsely. These craters have an area of roughly 10 to 50 μm2 and an average diameter of roughly 3 to 10 μm. The term “craters” as referred to herein is used in this context.
The generation of these craters affects all characteristics including the adhesive strength with the resin layer, acid resistance or anti-tin plating solution properties, peel strength, etching unevenness, and gloss level, and becomes an obstacle in achieving a fine wiring pattern. In particular, etching unevenness is undesirable because the abnormal appearance will become a problem.